1. Field of the Invention
The present invention relates to a method and apparatus for acquiring digital microscope images, and more particularly to a method and apparatus for acquiring a digital image of a specimen, or part thereof, on a microscope slide, which is larger than the field of view of the microscope.
2. Description of the Related Art
The use of digital images of microscope specimens, which are typically displayed on a display screen, is becoming increasingly important in the field of diagnostic pathology. For example, digital images of a microscope specimen may be captured using a microscopy imaging system and the acquired digital image data may be assembled together to form a composite image of the specimen, and transmitted to local or remote locations for diagnosis by a pathologist. Thus, the acquisition of digital images of pathology specimens advantageously enables specimens to be viewed by consultant pathologists at remote locations.
Conventional methods for acquiring a digital image of a microscope specimen which is larger than a single field of view image involves acquiring separate, adjacent field of view digital images or image “tiles” and subsequently mosaicing (also known as montaging or tiling) the adjacent images to obtain an overall composite image of the specimen or part thereof.
For example, WO-A-98/39728 discloses a method and apparatus for acquiring and reconstructing contiguous specimen image tiles. The adjacent image tiles are obtained by computer-controlled positioning of the microscope stage, on which the microscope slide is placed, beneath the imaging system comprising microscope and digital camera. The stage is driven by stepper motors, under control of a computer, in steps to precisely position the stage for acquiring each image tile, contiguous with the preceding image tile. Since the image tiles are contiguous, the image tiles can be readily seamlessly matched or abutted together to form a composite image.
Similarly, in EP-A-0 994 433, in the name of the present applicant, a method of acquiring digital microscope images is disclosed in which adjacent image tiles similarly are obtained by stepwise imaging of the specimen.
Whilst such conventional methods of acquiring digital microscope images result in a high quality composite image of the specimen, the imaging process is time consuming, particularly if imaging of a complete specimen is performed using a medium or high power objective lens in the microscope. This is because the time taken to image the complete specimen is proportional to the number of image tiles in the complete specimen. The higher the magnifying power of the objective lens, the smaller the area of the specimen contained in the field of view, and the greater the number of image tiles.
Moreover, the time taken to acquire an individual image tile is subject to physical constraints. In particular, for each image tile it is necessary to perform the sequential steps of: i) moving the microscope stage to the correct position; ii) pausing to allow the microscope stage to come to rest; iii) if necessary, focusing the microscope, and iv) operating the camera to acquire the image. To prevent excessive vibration or jitter of the stage during the stepwise movement, which would lead to poor quality images, the velocity of movement in step i) and the duration of the pause in step ii) are constrained by the physical limitations of the stage so that the time taken cannot be reduced below a certain amount.
In view of these limitations, imaging of a complete specimen of say 20 mm by 20 mm at 40× magnification may take over an hour and could take five to six hours. This time delay in the image of the specimen becoming available for diagnosis by a pathologist is problematic. In addition, the use of the valuable resource of the imaging system for such a lengthy period of time for only a single specimen may be difficult to justify.
One way of addressing this problem is to limit the area of the specimen to be imaged at high magnification to only selected portions. By restricting the area to be imaged at high magnification, it is possible to reduce the amount of time needed to acquire the specimen images. However, the selection of appropriate areas must be done by a skilled pathologist. In particular, it is necessary for an expert to identify the most significant regions of the specimen, to be viewed at high magnification, in order to make an accurate diagnosis.
If the pathologist is local to the imaging system, then the pathologist can select the relevant areas for scanning. However, this process represents a poor use of the pathologist's time. In addition, if a remote pathologist is then consulted for a second opinion, the remote pathologist will not be able to view, at high magnification, areas of the specimen other than those selected by the local pathologist. This compromises the ability of the remote pathologist to make an accurate diagnosis, since he or she is reliant on the local pathologist having selected all the important areas of the specimen. Many consultant pathologists will be reluctant to give a diagnostic opinion based on the limited images selected by another pathologist, particularly if that pathologist is not know to him or her.
One way of avoiding this problem, suggested in WO-A-98/44446, is to allow the remote pathologist to select the areas of the specimen for scanning at high magnification. However, this solution has the disadvantage that there will be a further delay between the time the remote pathologist makes the selection and the time when high magnification images become available due to the time taken for the specimen area to be imaged.
The method disclosed in EP-A-0 994 433 involves scanning the complete specimen only at high magnification. This overcomes the problem of only selected areas of the specimen being available for viewing by a pathologist at high magnification. However, as explained above, the process of acquiring adjacent high magnification images by stepwise movement of the stage is time consuming.
It would therefore be desirable to provide a method and apparatus for acquiring digital images of a microscope specimen at high magnification more quickly than conventional methods.